Interpretive Summary: In the process of photosynthesis, plants convert light into chemical energy. The energy produced by photosynthesis is then used to synthesize food, fuel and fiber; the products that are harvested as agricultural yield. Heat stress inhibits photosynthesis, reducing the overall yield of the plants. To determine how photosynthesis is affected by heat stress in a cool-season bioenergy crop, various photosynthetic parameters were measured at different leaf temperatures in Camelina and Camelina plants were grown under control and high temperatures. The data showed that photosynthesis and yield of Camelina were severely inhibited by moderate heat stress. The temperatures that caused this inhibition were only about 30-35ºC, much lower than the temperatures that caused heat stress in tobacco and cotton, two warm-environment species. Camelina plants did not acclimate to growth under conditions that imposed short intervals of moderate heat stress. Instead, seed yield was reduced by 63% under these conditions. The biochemical basis for this increased susceptibility of Camelina to heat stress was identified as the protein responsible for keeping the enzyme that captures carbon dioxide active. The results of this study provide new insights into the physiological and biochemical mechanisms that reduce the productivity of this bioenergy crop in warm environments. The insights gained can be used guide efforts to develop more heat-tolerant Camelina and other plant species.

Technical Abstract:
The temperature optimum of photosynthesis coincides with the average daytime temperature in a species’ native environment. Moderate heat stress occurs when temperatures exceed the optimum, inhibiting photosynthesis and decreasing productivity. In the present study, the temperature response of photosynthesis and the potential for heat acclimation was evaluated for Camelina sativa, a potential bioenergy crop. The temperature optimum of net CO2 assimilation rate (A) under atmospheric conditions was 30-32°C and was only slightly higher under non-photorespiratory conditions. The activation state of Rubisco was closely correlated with A at supra-optimal temperatures, exhibiting a parallel decrease with increasing leaf temperature. At both control and elevated temperatures, the modeled response of A to intercellular CO2 concentration was consistent with Rubisco limiting A at ambient CO2. Rubisco activation and photochemical activities were affected by moderate heat stress at lower temperatures in camelina than in the warm-adapted species cotton and tobacco. Growth under conditions that imposed a daily interval of moderate heat stress caused a 63% reduction in camelina seed yield. Levels of cpn60 protein were elevated under the higher growth temperature, but acclimation of photosynthesis was minimal. Inactivation of Rubisco in camelina at temperatures above 35°C was consistent with the temperature response of Rubisco activase activity and indicates that Rubisco activase is a prime target of inhibition by moderate heat stress in camelina. That photosynthesis exhibited no acclimation to moderate heat stress will likely impact the development of camelina and other cool season Brassicaceae as sources of bioenergy in a warmer world.